I bought several monthes ago a new Geforce FX5200.
I tried to use it with nvidia drivers on Suse without success but as I did a lot of manioulation, I feared that my behaviour could have prevented it from working.

When rebooting, when xserver starts, I get a blank screen.
To help you and following the procedure, I obtained the same behaviour by running a manual startx -- -logverbose 6.
Here are the logs :
old is for natural boot other one is for startx in verbose mode.

I uninstalled via Synaptic nvidia-glx and restricted-modules.
I installed the correct version (100.14.03) you advised and support.
Here are the two logs, one with normal boot (old) and one with verbose.
Problem persists.

Are you installing the drivers from the classic .run file or from any kind of DEBs that Ubuntu has?
Cause I own an FX 5200 and I use it with no such problem on openSuSE 10.2(with apic and acpi=on) with 9755 driver and 100.14.03 one and all the previous ones including 9631.
Also I had installed in the past Kubuntu 6.06 and I also didn't have such problems.
I haven't use your distro but I think Kubuntu is almost the same with Ubuntu.

My suggestion: Install the 9755 driver provided from NVIDIA's site, and use nvidia-xconfig in installation to configure your xorg.conf.
Note: You must install kernel-headers deb before in order to build the driver!

If the problem persists install another distro(openSuSE 10.2 really worths trying and works excellent with this card!!) cause your card generally works perfect under linux.

I installed the drivers from nvidia site.
I set the option NvAGP to 0 and now startx works for the first time ever.
So a great thanks at first !
Then setting NvAGP to 0 won't be a problem in terms of performance ?

Do you think that the fact I have to do so is related to my motherboard or bios ?